Nonaqueous-electrolyte secondary batteries, such as lithium-ion secondary batteries, lithium secondary batteries and sodium-ion secondary batteries, are secondary batteries having high energy densities and enabling high powers to output. For example, lithium-ion secondary batteries have been used mainly as a power source for portable electronic devices. In addition, lithium-ion secondary batteries are expected to serve as a power source for electric automobiles having been anticipated to prevail from now on. A lithium-ion secondary battery has a battery construction in which the positive electrode and negative electrode are made of an active material being able to sorb (or occlude) and desorb (or release) lithium (Li), respectively. Thus, the lithium-ion secondary battery is operated by lithium ions moving within an electrolytic solution disposed between the two electrodes.
In current lithium-ion secondary batteries, lithium-containing metallic composite oxides, such as lithium-cobalt composite oxides, have been used mainly as an active material for the positive electrode. As for an active material for the negative electrode, carbonaceous materials have been used primarily. A polar plate for the positive electrode, and another polar plate for the positive plate have been fabricated as follows. One of the active materials, and a binder resin, as well as a conductive additive, if needed, are dispersed in a solvent to make a slurry. The resulting slurry is coated onto a metallic foil serving as a current collector. The solvent is removed by drying to form a mixed-agent layer. Thereafter, the current collector with the mixed-agent layer formed is molded to a shape by compressing with a roll pressing machine.
Recently, developments of negative-electrode active materials, which possess charge and discharge capacities excelling the theoretical capacities of carbonaceous materials greatly, have been under way, for use in lithium-ion secondary batteries. For example, silicon-based materials, such as silicon or silicon oxides having higher capacities than do carbonaceous materials, have been investigated.
Silicon-based materials alloyed with lithium are able to possess such a high capacity as 1,000 mAh/g or more. However, when a silicon-based material, like silicon or silicon oxide, is used as a negative-electrode active material, the volume of the negative-electrode active material has been known to expand and contract as being accompanied by the sorbing and desorbing of lithium (Li) in the charging and discharging cycles of lithium-ion secondary battery. Such expansions and contractions of the volume of the negative-electrode active material apply loads to a binding agent performing a role of retaining the negative-electrode active material onto a current collector. The binding agent to which the loads have been applied causes declines in adhesiveness between the negative-electrode active material and the current collector, and destructions in conductive paths within a negative electrode. Moreover, the repetitive expansions and contractions of the volume of the negative-electrode active material generate distortions in the negative electrode. The distortions arisen in the negative electrode cause the negative-electrode active material to be miniaturized, and cause the negative-electrode active material to be eliminated from the negative electrode. Therefore, when a silicon-based material, like silicon or silicon oxide, is used as a negative-electrode active material, the expansions and contractions of the negative-electrode active material result in such a problematic issue that no battery performance, such as initial efficiency, capacity and durability which the silicon-based material is supposed to have inherently, is obtainable.
For the purpose of solving the above problematic issue, adding a buffer to a negative-electrode mixed-agent layer has been investigated in order to inhibit a negative electrode as a whole from changing volumetrically, thereby inhibiting cyclability of the negative electrode from deteriorating. The buffer herein inhibits the negative electrode from suffering volumetric changes resulting from expansions and contractions of the volume of a negative-electrode active material.
For example, in Patent Gazette No. 1, using a negative-electrode active material, in which Si particles and amorphous carbon had been composited, led to successfully upgrading a lithium-ion secondary battery in the cyclability. However, the negative-electrode active material according to Patent Gazette No. 1 had a structure in which the Si particles were buried in the amorphous carbon with low conductivity. Consequently, the present inventors believe the following as concerns over the negative-electrode active material: declines in the lithium-ion conductivity within the negative-electrode active material at the time of charging and discharging the lithium-ion secondary battery, and increments in the battery resistance being accompanied by the declines. Thus, using the negative-electrode active material, in which Si particles and amorphous carbon have been composited, results in such a possibility that an output characteristic of the lithium-ion secondary battery declines.    Patent Gazette No. 1: Japanese Patent Gazette No. 4281099